memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

In case of memcg_online_kmem() failure, memcg_cgroup::id remains hashed
in mem_cgroup_idr even after memcg memory is freed.  This leads to leak
of ID in mem_cgroup_idr.

This patch adds removal into mem_cgroup_css_alloc(), which fixes the
problem.  For better readability, it adds a generic helper which is used
in mem_cgroup_alloc() and mem_cgroup_id_put_many() as well.

Link: http://lkml.kernel.org/r/152354470916.22460.14397070748001974638.stgit@localhost.localdomain
Fixes 73f576c04b ("mm: memcontrol: fix cgroup creation failure after many small jobs")
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4037,6 +4037,14 @@ static struct cftype mem_cgroup_legacy_files[] = {
 
 static DEFINE_IDR(mem_cgroup_idr);
 
+static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
+{
+	if (memcg->id.id > 0) {
+		idr_remove(&mem_cgroup_idr, memcg->id.id);
+		memcg->id.id = 0;
+	}
+}
+
 static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
@@ -4047,8 +4055,7 @@ static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
 	if (atomic_sub_and_test(n, &memcg->id.ref)) {
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
-		memcg->id.id = 0;
+		mem_cgroup_id_remove(memcg);
 
 		/* Memcg ID pins CSS */
 		css_put(&memcg->css);
@@ -4185,8 +4192,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
 	return memcg;
 fail:
-	if (memcg->id.id > 0)
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
+	mem_cgroup_id_remove(memcg);
 	__mem_cgroup_free(memcg);
 	return NULL;
 }
@@ -4245,6 +4251,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 
 	return &memcg->css;
 fail:
+	mem_cgroup_id_remove(memcg);
 	mem_cgroup_free(memcg);
 	return ERR_PTR(-ENOMEM);
 }